High compression beam generating system especially for velocity modulated tubes



2 Sheets-Sheet 1 W. KLEIN ET AL HIGH COMPRESSION BEAM GENERATING SYSTEM ESPECIALLY FOR VELOCITY MODULATED TUBES 5 a WWW Sept. 22, 1959 Filed Nov. 28, 1956 saturafivn range F i g. 3

INVENTOR W. KLE I N W. FR lZ ATTORNEY HF input focusing field Sept. 1959 w. KLEIN ETAL 2,905,847

HIGH COMPRESSION BEAM GENERATING SYSTEM ESPECIALLY FOR VELOCITY MODULATED TUBES Filed Nov. 28, 1956 2 Sheets-Sheet 2 Fig. 5

INVENTOR W. KLEIN W. F'RlZ ATTORNEY United States Patent HIGH COMPRESSION BEAM GENERATING SYS- TEM ESPECIALLY FOR VELOCITY MODULATED TUBES Werner Klein, Korntal, Wurttemberg, and Walter Friz, Stuttgart, Germany, assignors to International Standard Electric Corporation, New York, N.Y., a corporation of Delaware Application November 28, 1956, Serial No. 624,856

Claims priority, application Germany December 8, 1955 1 Claim. (Cl. 313-79) For the purpose of obtaining a low cathode load and electrode rays having a small beam diameter beam generating or gun systems of a high compression are required. To this end beam generating systems have already been proposed in which the emission surface diameter is a multiple of the beam diameter. In one form of beam generating system the reduction of the beam diameter is achieved in a space free of magnetic fields with the aid of purely electro-static means. At the point of the smallest beam diameter the beam enters an axial magnetic field. The transition from the magnetic field free space within the beam generating system to the full magnetic field intensity effected in a gap asshort as possible. This is accomplished in that the beam generating system is shielded by a ferromagnetic coat which, at the same time acts as a pole shoe, at the beam exit point, providing a rapid field increase. An aperture with about double the beam diameter size is provided for the beam passage. With this type of beam generating systems it is possible to focus electron rays of a high current density with the lowest possible expenditure of magnetic field intensity. These systems, however, have the disadvantage that the noise values of tubes so equipped will grow inadmissibly high.

Furthermore, the noise depends to a very great extent on the magnetic field intensity for focusing the electron beam. Since the noise value strongly increases with an increasing magnetic field intensity, and since the beam focusing is insufiicient when the magnetic field is weak, there is only a very narrow range of magnetic field intensity usable to obtain acceptable values with respect to the focusing and the noise. Accordingly, the tube is very critical in regard to the adjustment to these operating values. For this reason and because of the variation of the values of the components, interchangeability of tubes is hardly possible. Another disadvantage of these kinds of beam generating systems consists of the extreme sensitivity to the slightest deviations from the electrode dimensions and with respect to the positioning of the electrodes with respect to one another, especially the arrangement of the beam generating system in relation to the exit aperture provided in the shielding.

In another kind of beam generation the full magnetic flux passes through the emissive surface of the cathode. From the initial emission out of the cathode surface the electrons move along the magnetic field lines. If a compression of the beam is supposed to be achieved with this kind of beam generation it is necessary to adapt the magnetic field lines to the electron paths or tracks. This can be accomplished, for example, by employing an arched-type cathode and a magnetic field converging from the cathode surface.

For guiding the thus produced beam, however, relatively high magnetic field intensities are necessary. The exact tuning or adjustment of the magnetic and the electric field within the zone of the beam generating system is rendered extremely difficult especially when a high beam compression is required.

For obtaining a high compression in the beam generating system and at the same time achieving good noise properties it is proposed by the invention to use a beam generating or gun system with a finite magnetic field in the cathode plane, the axial magnetic field component of which gradually increases from the cathode to the full magnetic field value. According to the invention, by way of example, this may be accomplished in that the operated shield diaphragm surrounding the beam generating system is separated by a magnetic gap from the remainder of the beam shield. This construction gives the added advantage that this diaphragm, at the same time, can also be used as an electrode of the beam generating system with a potential that may be chosen at will.

The above-mentioned and other features and objects 01'' this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a diagrammatic sectional View of a tube incorporating this invention;

Figs. 2 and 3 are curves used in explaining the advantages obtained by the invention;

Fig. 4 is a diagram showing the magnetic lines of force about the electron gun shield, and

Fig. 5 is a curve showing the degree of penetration of the magnetic field within the shield.

The advantage of the arrangement according to this invention will become apparent from a study of Fig. 4 of the accompanying drawings, showing a field line diagram of such an arrangement. The magnetic gap existing be tween the ferro-magnetic cover plate 11 and the ferromagnetic coating or shielding 12, serves to separate these parts so that they, when inserted into an outer magnetic field, will vary it in the manner illustrated in Fig. 4. The coating 12 and the cover plate 11 may have difierent potentials applied thereto. By selecting the cross-sectional contour and the position of the magnetic gap the field pattern may be varied within a large range and adapted to the requirements of the beam guidance within the partially shielded space. In particular, the contour of the magnetic potential surfaces can be designed in such a way that they will form normal surfaces with respect to the desired direction of flight of the electrons.

The noise developed in a tube incorporating a beam generating system according to the invention is smaller than those currently used and independent of the magnetic field intensity. While with the known arrangements employing shielded beam generating systems without a magnetic gap there is achieved a noise factor of 30 db using a small magnetic focusing field intensity, the noise factor of tubes having a beam generating system with a magnetic gap is at least 4 db lower. A further advantage of the beam generating system of the invention is that the beam is substantially less sensitive with respect to focusing when the tube is operated at the upper limit of efiiciency. In the case of tubes with a shielded beam generating system and without a magnetic gap the focusing, a small magnetic field intensity is necessary for a not too high noise, and at a full modulation of the tube the focusing is substantially deteriorated by a strong increase of the current in the delay helical coil. This is not the case with the arrangement according to the invention, because the coil current only rises slightly even at a full modulation of the tube.

In Fig. 1 there is shown a highly compressing beam generating system according to the invention. The electron beam from cathode 1 is compressed with the aid of the focusing electrode 2 and the accelerating anode 3. The convergence angle 0 is assumed to be 30 in the example. The compression of the electron beam, with Patented Sept. 22, 1959 p the aid of such a type of beam generating system amounts to about 25-30, considering the cathode surface in proportion with the cross-section of the beam in plane 4. By means of the cylindrical shielding 5 the beam generating system is shielded from the magnetic field produced by the coil 10. Diaphragm 6 constitutes a pole shoe permitting the magnetic field to rise from the small field amount Within the magnetic shielding to its nominal value in the drift space 8 within a short as possible stretch. Within the drift space S the electron beam is guided or conducted by the magnetic field. The magnetic gap 7 between the shielding 5 and the diaphragm 6 serves to provide the necessary field intensity in the cathode plane, which substantially improves the noise of a tube incorporating such a system.

This magnetic gap may also be established by inserting a non-magnetic material, such as ceramic, copper, mica, etc. between the shielding and the diaphragm; although the insertion of non-magnetic material between the shielding and the diaphragm substantially facilitates the assembly of the shielding, it is also possible to arrange the diaphragm and the shielding without any insertion.

An arrangement as shown in Fig. 1 with the following dimensions has proved to provide the advantage of the invention. The thickness of the diaphragm amounts to 1 mm. and the diaphragm has an external diameter of 14.1 mm. and a diaphragm aperture of 3 mm. The width of the magnetic gap between the shielding with an external diameter of 17.5 mm. and the diaphragm disk is determined by a sheet metal ring of 0.25 mm.

Fig. 5 shows the magnetic field intensity plotted as percentage along the axis of the ferromagnetic shield and diaphragm.

In a traveling wave tube with a delay coil voltage of 1400 volts and a beam current of 35 mm. with a beam generating system resembling the one shown in Fig. 1 but without a magnetic gap, a noise factor as is plotted as ordinates in Fig. 2 as curve 1 against the focusing field intensity as abscissas was obtained. Curve 1a shows the measured coil current for this tube. If a maximum coil current portion of 2 ma. is admissable and the maximum admissable noise factor is, for example, 38 db then there will result a range of only about 10% of the focusing field intensity (550-600 gauss), within which there will be obtainable results for the noise and the focusing.

The noise factor of a traveling wave tube with the same operating data and with a beam generating system 4 according to the invention (Fig. 1) will remain practically independent of the focusing field intensity, as may be seen from curve 2 in Fig. 2. The coil current is shown by way of the curve 2a.

In Fig. 3 there is shown the noise factor of a travelling wave type tube comprising an almost completely shielded beam generating system, plotted against the high frequency input power. The noise factor of a tube comprising the substantially completely shielded beam generating system, as is shown by curve 1, rises up to the saturation range of the tube by about 1015 db, While the noise factor, in the case of a tube employing the inventive beam generating system only increases by about 4-5 db.

While the principles of this invention have been described in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention as set forth in the objects thereof and in the accompanying claim.

What is claimed is:

An electron gun system for producing a highly compressed electron beam by the use of magnetic focusing comprising an emitting cathode of a given area, electrostatic beam concentrating and accelerating means, a magnetic shield comprising a cylinder extending over said cathode and said electro-static means, an apertured diaphragm of magnetic shielding material positioned at one end of said cylinder with its aperture aligned with said beam, the outer edge of said diaphragm being spaced from said magnetic shield to provide an opening therebetween, whereby a controlled amount of the magnetic field enters the shielded area and means external of said magnetic shield for controlling said beam externally thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,225,901 Bruche Dec. 24, 1940 2,267,083 De Gier Dec. 23, 1941 2,431,077 Poch Nov. 18, 1947 2,638,561 Sziklai May 12, 1953 2,707,758 Wang May 3, 1955 2,729,759 Kratz et a1. Jan. 3, 1956 2,733,364 Flory Jan. 31, 1956 2,797,353 Molnar et al. June 25, 1957 

